Currently, a lithium-ion battery (LIB) is one of the most promising battery technologies that can provide higher energy density than other batteries. It also does not suffer from the memory effect and the loss of charge is relatively slow when not in use. Hence, high-performance LIB remains the preferred technology that would address a much broader range of energy source/storage for a variety of applications if advanced cathode material with extreme operating capability could be realized.
Current lithium ion batteries mostly utilize metal oxides as cathode material with LiCoO2 as the most popular and commercially successful representative [2]. However, due to the intrinsic material properties of these metal oxides, further enhancement of LIB performance is limited. Specifically, the metal oxides have limited average potential versus Li/Li+, mostly well below 4V except LiMn2O4, and most of the metal oxides have the specific capacity well below 180 mAh/g, with the exception of LiNiO2. The metal oxides are also “hot” cathode materials due to the thermal runaway reaction, so there is also a concern for safety.
Another major group of cathode materials is LiMPO4, where M=Co, Ni, Mn, or Fe. These materials have the electrode potential in the range of about 3.5-5.2 V, but the capacity is still limited below 150-170 mAh/g [3]. Further, these materials have poor electrical conductivity, so they have to be made in the form of tiny nanoparticles and coated with a carbon layer, which increases the cost of the materials.
The Li2MSiO4 silicate family (where M=Co, Fe, or Mn) has attracted research activities for the applications in LIB only recently [4, 5] and much work needs to be done to thoroughly understand its properties. The most significant advantage of this group of materials is the polyanionic structure with two lithium ions per formula unit. The theoretical capacity of these materials is as high as about 330 mAh/g. Unfortunately, Co is an expensive metal despite its high average voltage of about 4.3 V. Therefore, pure Li2CoSiO4 is not an efficient and economic way for making a cathode. Li2FeSiO4 has good cycle-ability, but the average voltage is only about 3.1 V, far below 4 V. On the contrary, Mn is an inexpensive and abundant element. The average voltage of Li2MnSiO4 is about 4.2 V. The reported specific capacity of Li2MnSiO4 is about 210 and about 250 mAh/g at room temperature and 55° C., respectively [6]. However, it has to be noted that the entire family of Li2MSiO4 silicates has poor electrical conductivity, therefore Li2MnSiO4 has to be made into nanoparticles and coated with carbon in order to improve the conductivity, similar to the aforementioned LiMPO4. The additional carbon-coating process is expensive.
Another major drawback of Li2MnSiO4 is its poor cycle life characterized by the poor capacity retention and rate performance. A recent report shows a 50% retained capacity at room temperature after 20 cycles. The poor cycling performance is mainly attributed to Jahn-Teller distortion, structural instability and low electronic conductivity of the material. Another possible attribution is the electrolyte degradation.
The presence of Mn3+ ions in the material system is responsible for the dynamic Jahn-Teller distortion and manganese dissolution, a situation similar to that of LiMn2O4 spinel cathode. Also, the structure of Li2MnSiO4 is prone to collapsing upon delithiation. During delithiation, a phase separation into MnSiO4 and Li2MnSiO4 may occur, leading to the formation of an amorphous structure, which in turn results in the drop of reversible capacity of the electrode during the cycling.
An effective way to minimize the dynamic Jahn-Teller distortion and prevent the collapse of Li2MnSiO4 structure is the utilization of a solid solution of Li2MnSiO4 and Li2FeSiO4 as the cathode. However, there is very limited research available on this topic. A few literature reports do show that addition of Li2FeSiO4 has prevented Li2MnSiO4 from collapsing during delithiation [7, 8]. Nonetheless, according to the reports, the cyclic reversibility is still unacceptable.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.